1. Field of the Invention
The present invention relates to an imaging apparatus which has an image stabilizing function.
2. Description of the Related Art
An imaging apparatus typified by a still camera or a video camera has an optical image stabilizing system or an image sensor image stabilizing system available as a system for correcting vibrations such as camera shakes applied to the apparatus from the outside.
These systems perform digital signal processing on a signal from a vibration detection sensor which detects a vibration degree, via an analog-to-digital (A/D) converter, calculate a vibration correction amount to execute digital-to-analog (D/A) conversion, and then drive a correction unit for image stabilizing, i.e., a shift lens or an image sensor.
An angular speed sensor is often used for vibration degree detection. This angular speed sensor vibrates a vibration material such as a piezoelectric element at a constant frequency, and converts Coriolis force generated by a rotational motion component into a voltage to obtain an angular speed signal.
As an apparatus for performing A/D conversion, digital signal processing, or D/A conversion, a microcomputer is used which includes a filter for cutting off a plurality of predetermined frequencies and an integration filter. Recursive digital filters are available as such filters. The recursive digital filter includes a feed-forward unit and a feedback unit.
FIG. 9A is an overall block diagram of a recursive primary digital filter. The recursive digital filter includes a feed-forward unit and a feedback unit. In the recursive digital filter, an intermediate value is a calculation result of the feedback unit. In this case, an intermediate value Z[n] is obtained at current sampling. A value after passage through a delay element Z−1 was obtained at last sampling. This delay element determines a digital filter order.
FIG. 9B illustrates the feedback unit cut out from the recursive digital filter. An intermediate value Z[n] of current sampling is calculated from an input value X[n] of the current sampling and an intermediate value Z[n−1] of last sampling, where n denotes a number of sampling times. FIG. 9C illustrates the feed-forward unit cut out from the recursive digital filter. An output value Y[n] of the current sampling is calculated from the intermediate value Z[n] of the current sampling and the intermediate value Z[n−1] of the last sampling. FIG. 9D illustrates an operational expression when gains of the feed-forward unit and the feedback unit are respectively set to constants a, b, and c.
FIG. 9E is an overall block diagram of a nonrecursive primary digital filter. Unlike the recursive digital filter, the nonrecursive digital filter includes only a feed-forward unit. An input value X[n] is obtained at current sampling, and an input value X[n−1] of last sampling is an intermediate value in the nonrecursive digital filter. In other words, in the nonrecursive digital filter, a value after passage through the delay element Z−1 is an intermediate value. FIG. 9F illustrates an operational expression when gains of the feed-forward unit are set to constants a and b.
To obtain a filter having desired characteristics, values and signs of the constants a, b, and c are appropriately set. By setting these constants appropriately, a digital high-pass filter or a digital low-pass filter can be realized.
Secondary or higher-order digital filters are realized by increasing delay elements Z−1. The number of intermediate values is increased according to an order.
The optical image stabilizing system corrects image vibrations on the image sensor (removes image vibrations from an image formed on the image sensor) by moving the shift lens which is a correction unit for image stabilizing within a plane orthogonal to an optical axis by a vibration correction amount. The image sensor image stabilizing system corrects image vibrations on the image sensor by moving the image sensor which is a correction unit for image stabilizing within the plane orthogonal to the optical axis by a vibration correction amount. The present invention described below can be applied to both systems, and thus the optical image stabilizing system will be described below as a representative example.
In the imaging apparatus having the image stabilizing function of the aforementioned system, a shift lens drive unit is instructed to move by a vibration correction amount. When the shift lens that is a control target reaches a driving target position, a real position of the shift lens is obtained. Feedback control is performed to reduce a deviation between the driving target position and the real position to zero.
A driving range of the shift lens in the optical image stabilizing system is determined by a mechanical limit or a limit of optical performance. When enabled, the image stabilizing function corrects also vibrations caused by a panning operation in addition to camera shakes. Consequently, an image may not be stabilized correctly when the shift lens is near a driving limit. Thus, Japanese Patent Application Laid-Open No. 7-199263 discusses a technique of returning a shift lens to a predetermined position within a driving range before exposure when a driving amount of the shift lens is large.
According to the technique discussed in Japanese Patent Application Laid-Open No. 7-199263, the shift lens is returned to the predetermined position before exposure when the driving amount of the shift lens for image stabilizing is large. Thus, a driving range can be secured even when the shift lens is near the driving limit.
When the digital low-pass filter is used, however, if an intermediate value is large even when a vibration correction amount which is an output of the digital filter is changed by a predetermined amount, the output is increased immediately after the change is made by the predetermined amount. As a consequence, the driving amount becomes large again within a short time. When the intermediate value is large, the image may not be stabilized correctly because the intermediate value of the digital low-pass filter works to reduce a final output of the digital filter.